1. Field of the Invention
This invention relates in general to refrigerant expansion devices. More specifically, it relates to expansion devices that have a variable expansion area that is operated by the pressure differential existing between the high pressure and the low pressure side of a refrigeration system.
2. Description of the Prior Art
A compression refrigeration system comprises: a compressor, a condenser, an expansion device and an evaporator connected in a closed circuit to provide refrigeration. Hot compressed refrigerant vapor from the compressor enters the condenser, where it transfers heat to an external heat exchange medium and condenses. Liquid refrigerant, at a high pressure flows from the condenser to the expansion device, where the refrigerant under goes a pressure drop and at least partially flashes to a vapor. The liquid vapor mixture then flows through the evaporator where it evaporates and absorbs heat from the external surroundings. The low pressure refrigerant vapor then returns to the compressor to complete the circuit.
Although the expansion device is often a simple construction, its role in the refrigeration system is crucial. Ideally, the expansion device should meter refrigerant in a manner such that refrigerant leaving the evaporator is super-heated by a controlled, relatively small amount. The foregoing is desired to prevent any damaging liquid or refrigerant from entering the compressor and to avoid subjecting the compressor to excessive temperature s from highly supper-heated vapor. The performance of the expansion device plays an important role not only in protecting the compressor, but also in determining the cooling capacity of the refrigeration system. Since the system is a closed circuit, any effect the device has on the low or evaporator side is intimately tied in with the performance of the high or condenser side. Most conventional air conditioning systems incorporating compression refrigeration units of the kind described are designed to have a predetermined cooling capacity at a given ambient temperature. The capacity of the system usually decreases at ambient temperatures above the design point. The decrease in capacity of the system at temperatures above the design point is directly related to the type and design of the expansion device.
Among the most commonly used expansion devices are thermostatic expansion valves, capillary tubes and other fixed orifice devices. Thermostatic expansion valves control the flow rate of liquid refrigerant entering the evaporator as a function of the temperature and pressure of the refrigerant gas leaving the evaporator. This control is achieved by varying the cross-sectional area through a needle type valve contained within the valve body. The needle is typically joined to a flexible metal bellows or diaphragm which is, in turn, actuated by a non-heat conducting rod connected at its other end to a sealed bellows. The sealed bellows, in turn, is joined to a thermostatic sensing bulb by means of a capillary tube. This bulb provides feedback to the valve relating to the temperature of the refrigerant leaving the evaporator and the valve responds by increasing or decreasing the flow of refrigerant through the needle valve according to this feedback. While being highly efficient in their operation and readily responsive to changes in load upon system to vary the flow of refrigerant to the evaporator, thermostatic expansion valves are also complicated and relatively expensive. Further, in split system type air conditioning systems, wherein the compressor and condenser are located outside at a remote location from the evaporator, the distance of the sensing bulb from the compressor results in less than optimum conditions in such systems.
Capillary tubes are quite often used in place of thermostatic expansion valves, particularly in smaller applications, wherein ambient air is almost universally utilized as the condensing medium. Although capillary tubes are relatively inexpensive to manufacture and are simple to install, they have some serious operating limitations, particularly when they are operating at conditions above or below the design point of the system.
As an example, as the outdoor ambient temperature increases there is an increase in the pressure differential across the expansion device and the compressor. At higher pressure ratios the compressor pump a lower flow rate, however, a capillary type expansion device at such higher pressure ratio passes a higher flow rate. This flow imbalance results in the loss of stored sub-cooled liquid refrigerant from the high side and loss of superheat and flooding of the low side. These phenomenon greatly reduce system efficiency and performance.
An additional problem is found at relatively low ambient temperatures, wherein the pressure differential across the expansion device is of a relatively small magnitude. Under these conditions the subcooling of the refrigerant entering the expansion device increases as the outdoor ambient temperature falls, leading to more refrigerant being stored in the condenser, which starves the evaporator. As a result, more of the evaporator becomes filled with superheated vapor and the superheat leaving the evaporator increases. Using increased evaporator surface to superheat refrigerant is not putting the surface to its most effective use.
Another known fixed orifice expansion device is the orifice plate. Very simply, an orifice plate comprises a thin plate having an expansion orifice extending therethrough. Orifice plates are small and inexpensive, but they are erratic in performance. Hence, such plates are not in wide use.
In an attempt to provide an alternate expansion device, having the economical advantages of the capillary tube, while being smaller and more efficient in operation, the expansion device described in commonly assigned U.S. Pat. No. 3,642,030 entitled "Refrigerant Throttling Device" and issued on Feb. 15, 1972 in the name of Larry D. Amick was developed. That device comprises a body member having a tubular insert having prescribed length-to-bore diameter ratios, a conical inlet, and a conical exit.
Continuing efforts to develop an economical, efficient and effective fixed orifice expansion device resulted in the development of the expansion device described in commonly assigned U.S. Pat. No. 3,877,248 entitled "Refrigerant Expansion Device" which issued Mar. 1, 1974 in the name of Fred V. Honnold, Jr. That device comprises a body having an expansion conduit extending therethrough with a flat entrance presenting a sharp edge orifice to incoming refrigerant. The sharp edge entrance orifice effects a major portion of the refrigerant pressure drop at the entrance, so that only a short conduit length is necessary to effect the balance of the pressure drop demanded of the device. The body member is incorporated in a unitary coupling member configured to join refrigerant lines from the condenser to the evaporator.
Commonly assigned U.S. Pat. No. 3,992,898 entitled "Moveable Expansion Valve" which issued Nov. 23, 1976, in the name of Richard J. Duell and John A. Ferrel represents a further refinement of a fixed orifice expansion device. In the device of this patent, the refrigerant metering port is formed in a free floating piston which is mounted within a chamber. When refrigerant flows through this device in once direction, the free floating piston moves to one position wherein the refrigerant flow is through the metering port thereby serving as an expansion device. When refrigerant flows through this device in the opposite direction, the free floating piston moves to a second position, wherein refrigerant is allowed to flow through a number of flow channels formed in the outer peripheral surface of the piston to thereby allow substantially unrestricted flow through the device. This arrangement allows such a device to be used, in combination with a second expansion device of the same design in a heat pump system to allow the desired expansion of the refrigerant through the system flowing in both the cooling and heating directions.
In a cooling only system the expansion device of the U.S. Pat. No. 3,992,898 allows a system to be adjusted as to the amount of refrigerant superheat and other expansion parameters by changing the piston contained within the valve body in the field. The piston usually is changed to match the diameter of the metering port, running the length of the piston, with the requirements of a particular system to optimize performance.
U.S. Pat. No. 4,263,787, issued to the assignee hereof, entitled "Expansion Device with Adjustable Refrigerant Throttling" which issued Apr. 28, 1981 to Albert A. Domingorena relates to an improvement of the device of the U.S. Pat. No. 3,992,898 which allows adjusting the diameter of the metering port without having to break into the refrigeration circuit of the system to change the piston.
Summing up the state of the prior art, thermostatic expansion valves, while being highly efficient in their operation and readily responsive to changes in load upon the system to vary the flow of refrigerant to the evaporator, are complicated, expensive, and have drawbacks in certain applications. For this reason they are generally not employed in small applications. As a result, capillary tubes or other fixed orifice expansion devices are generally used in small applications. Such devices are relatively inexpensive, however, as discussed above, they have operating limitations at both high and low ambient temperatures.
From the foregoing, it is evident that the need exists for a refrigerant expansion device which is inexpensive to manufacture and which is effective in performance over a wide range of operating conditions.
Commonly assigned U.S. Pat. No. 5,031,416 entitled "Variable Area Refrigerant Expansion Device Having a Flexible Orifice" which issued Jul. 16, 1991 in the name of Alan S. Drucker and Peter L. Cann discloses such a device. In the '416 patent a refrigerant expansion device is provided wherein the flow metering port is formed an elastomeric element and the elastomeric element deforms in response to an increased pressure differential across the device to decrease the size of the flow metering port. It has been found that in operation, at high pressure differentials, the size of the refrigerant expansion orifice according to the '416 design will change in size as a result of a net axial force on the expansion element which is proportional to the difference between the high pressure and the low pressure side of the refrigeration system.